1. Field of the Invention
The present invention relates to stable aqueous emulsions of hydrolyzed silanes and their application to various substrates to impart water repellency and lubricity. More specifically but not by way of limitation, the present invention relates to aqueous emulsions of a silane and an effective amount of an emulsifier having a hydrophile-lipophile balance, HLB, sufficiently high to retain the silane in a stable aqueous emulsion in substantially a hydrolyzed state.
2. Description of the Related Art
It is known that hydrolyzable silanes can be applied to surfaces to impart hydrophobic or water repellency properties (see, B. Arkles, Chemtech, 1977, 766). These silanes are applied to the surfaces either dissolved in a volatile organic solvent or as aqueous solutions or emulsions. With the solvent based products, the solvent must be evaporated once the solution is applied to the surface. In addition, the hydrolyzable silane must be contacted with water or sufficient adsorbed moisture on the surface being treated to hydrolyze the silane so that it may chemically bond to the surface to form a durable coating (see for example, U.S. Pat. Nos. 4,342,796 and 4,525,213). The presence of volatile solvents in coating formulations is generally harmful to the environment and may be hazardous due to their flammability. In addition, treatments of surfaces with these hydrolyzable silanes frequently require the use of elevated temperatures and certain catalysts to accelerate the hydrolysis of the silane and the condensation with the surface to achieve the desirable effects (see for example, U.S. Pat. Nos. 4,478,911 and 4,874,431).
When the hydrolyzable silanes are applied from the more preferred aqueous solutions or emulsions, buffers must be added to the aqueous mixtures to maintain the pH of the mixture within narrow limits to prevent premature hydrolysis of the silane and subsequent self-condensation to a polymeric polysiloxane (see for example, U.S. Pat. Nos. 4,889,747 and 4,990,377). Such a polymeric structure is no longer dispersible in the aqueous medium, and contains a reduced number of active sites that can bond the silane to the substrate. Generally acidic or basic conditions will accelerate this hydrolysis. However, if the aqueous emulsion is stabilized with a buffer to prevent hydrolysis on storage, this buffer must be overcome once the silane is applied to the substrate to now permit the hydrolysis to take place, and bonding with the substrate to occur. The prior art therefore teaches that for the preferred aqueous systems, the silane must be retained in the emulsions in a non-hydrolyzed state to achieve adequate storage stability, and that hydrolysis must then be caused to take place when the silane is applied to the substrate so that the proper bonding can occur between the silane composition and the surface to produce the desirable properties.